1. Field of the Invention
This invention pertains to the field of radiation measurement and more particularly, to a radiation detector and an associated method for accurately determining the location of, and measuring the degree of, very low level radiation.
2. Background Information
The current state of the art in radiation detectors may be considered first with respect to existing hand-held radiation detectors which are mainly manufactured for field survey use or to monitor radioactive spills or contamination. Because of these objectives, such existing detectors have been designed with a large sensitive area, and the need for sensitivity to very minute amounts of radioactivity is not very critical. Also, such detectors, being designed to be sensitive to radiation arriving at the sensitive area from all directions, give only a very rough idea of source location.
The context of the present invention is the field of biological studies, and for such purposes the need for high sensitivity and good localization is very important. Many times in such biological studies or experiments, one has only a minute amount of samples, or the biological samples can only tolerate or incorporate a small amount of radioactive labels or traces. In such cases, high sensitivity is clearly necessary. Moreover, adequate spatial resolution is required, for example, to localize radioactive lines in biology experiments like Southern blotting or hybrid-dots.
Accordingly, it is a primary object of the present invention to provide a hand-held radiation detector with sufficient spatial resolution.
An accompanying object is to provide such radiation detector which additionally possesses a high degree of sensitivity.
In fulfillment of the above stated objects, the primary feature of the present invention resides in a unique field focusing arrangement which results in a high uniform sensitivity over the entire entrance window of the chamber. This advantageous result is achieved because of three significant parameters that have been judiciously selected: the design of the point of the needle; the anode in the form of such needle has been situated or disposed a suitable distance from the radiation window; and a metallic tubular member, which defines the upper part of the radiation detector housing, is in contact with the metallized portion of the radiation window. Together, the metallic tubular member and metallized window portion form the cathode.
The electric field lines from the central region of the entrance window connect with the part of the point of the needle called the amplifying region (AR). Without the conducting tubular member, electric field lines emanating near the edge of the metallized window, i.e., away from the center of the window, would normally fall outside the amplifying region (AR) of the needle, and therefore be away from the point of the needle and along the body of the needle. Because of the conducting tubular member, the electric field lines from that tubular member push the electric field lines from the edge of the window into the region (AR) of the needle. As a result of this bunching of the electric field lines from the total area of the entrance window into the amplifying region (AR) of the needle, the chamber has uniform sensitivity over all the surface area of the window. If the needle were to be positioned too close to the window, the field lines from the tubular member would redistribute themselves along the body of the needle and the field from the edge of the window would also move away from the region (AR) of the needle. If the needle were to be positioned too far from the window, the electrons released by the radiation to be counted will have a long drift path to the region (AR) of the needle. The longer the drift path, the less likely the electrons will survive the trip to the needle, and the efficiency of the chamber will decrease. The high efficiency of the chamber is achieved by the optimum positioning of the needle with respect to the window, as will be explained herein. Consequently the drift path from the window to the needle is minimized, and one still has all the field lines from the window falling into the amplifying region (AR) of the needle.
The gas combination has also been chosen to optimize the efficiency of the chamber. Argon was chosen as the main gas because it is easily ionized by the incoming radiation but it is very stable because it is a noble gas. However, argon alone is not sufficient because when electrons multiply and avalanche in the field (AR) of the needle, a copious amount of X-rays are produced which can interact somewhere in the chamber and initiate another avalanche thereby giving spurious counts. To correct this unwanted effect, a small amount of quenching agent is added, in this case carbon-dioxide gas (CO.sub.2). The ratio found for optimum performance was argon (90-95%) and CO.sub.2 (5-10%) by volume.
It should be pointed out, in order to place the invention in its proper context and to provide background information that will enable full appreciation of the invention, that there has been prior knowledge with respect to needle counters or detectors. For example, reference may be made to (I) Y. Fuchita et al "Nuclear Instruments and Methods" 128, 523 (1975). Such article describes what is referred to as a "small" gas-flow type Geiger counter, which is cylindrical in shape and has a needle-tip (anode) against an end-window plane (cathode), and was developed particularly for low level, beta counting of solid sources.
The end window therein comprises a gold-plated mylar film, and the needle is surrounded by an insulating tube with the tip projecting beyond the tube end into the open space of the chamber. A high voltage connector is connected to the assembly, which includes a copper body, and a gas inlet and outlet passageways are provided for continuous flow of gas into and out of the detector.
Although the electric field configuration within the chamber of the Fujita device is not specifically illustrated in the referenced article, it will be understood that a large fraction of the field lines end up at the side of the needle. Consequently, electrons following these field lines will not avalanche sufficiently to produce the desired electron multiplication which is necessary to achieve sufficient sensitivity for the purposes already described.
It should also be especially noted that the chamber in the Fujita et al device is operated in the Geiger-Muller mode using an He-isobutane gas mixture. As a result of operation in this Geiger-Muller mode, the dead time of the chamber is about one millisecond. Consequently, a source which has more than 60,000 DPM (decays per minute) cannot be read accurately by this type of detector.
Another reference that may be useful as background material is an article by C. Grunberg and J. LeDevehat, NUCLEAR INSTRUMENTS AND METHODS, 118 (1974), pages 457 to 463. Described therein is a multineedle detector designed to give two-dimensional position information. However, the described detector does not have a means for focusing the field so as to achieve sufficient electron amplification or multiplication. Its electric field configuration may be appreciated, for example, by referring to FIG. 4 of the article. As indicated in the explanation of FIG. 4, which is a view of the multiplication area in the needle chamber, only a part of the electric field lines are in the multiplication bulk, so that an efficiency loss occurs.
A third reference that may be referred to for background information is an article by G. Comby et al, NUCLEAR INSTRUMENTS AND METHODS, 174, 77 (1980) pages 77-92. Such article is primarily concerned with applications of a multineedle detector. However, there is disclosed an elementary detector, which is referred to as a ring counter. This ring counter is depicted in FIG. 1A of this article and apparently involves a disc-like cathode for the purpose of focusing the electric field lines.
Accordingly, whatever the merits of the several reference detectors discussed above, they do not provide for the efficient electron multiplication obtained by the present invention and, therefore, are not suitable or appropriate for the purpose of biological studies involving measurement of very low level radiation.